1. Field of the Invention
The present invention relates to a thermosetting powder paint composition, and more particularly, relates to a thermosetting powder paint composition which provides a baked film having excellent external characteristics (smoothness, sharpness and the like), physical properties (impact resistance, mar resistance, adhesion and the like), weatherability, ultraviolet ray resistance and chemical properties (acid resistance, solvent resistance and the like), provides a paint which is excellent in storage stability, and has low temperature melting property/low temperature curing property.
2. Description of the Related Art
[Trend of Investigation and Development in Technical Filed of Paint and Expectation for Powder Paint from the Viewpoints of Ecology and the Like]
A solvent-borne paint has been conventionally used for painting an article, and in order to be used in a field wherein strict qualities are required such as an automotive field, a paint satisfying various requirements has been developed and used.
Recently, in the technical field of paint, conversion to a powder paint from a solvent-borne paint has been expected, from the viewpoints of local or global environmental safeguards, improvement of occupational safety and hygiene environment, prevention of fire and explosion, saving resources and the like. According to historical and social requirements, with increase of expectation for high performance and variation of a powder paint, the powder paint has also been required to have high-grade film properties (for example, impact resistance, acid rain resistance and the like) corresponding to those of the solvent-borne paint.
Though the powder paint is thus required to have more strict film properties, a powder paint completely satisfying these requirements has not necessarily been commercially available.
[Technical Background of General Powder Paint]
As specific examples of a conventional-type powder paint, there are listed, for example, polyester resin powder paints and epoxy resin powder paints mainly composed of a bisphenol A. However, these paints have problems not only in weatherability but also in resistance to acid rain which has recently become an environmental problem. Therefore, these paints have problems when used outdoors like paints for automotive bodies.
[Technical Background of Acrylic Resin-based Powder Paint]
Japanese Patent Application Laid-Open (JP-A) No.49-34546(1974) discloses a paint which is cured by reacting an acrylic resin component having a glycidyl group with an aliphatic dibasic acid (a curing agent component). However, the above-described powder paint does not necessarily have sufficient curing speed, therefore, high temperature and long time baking conditions are indispensable. And, a film formed from the above-described powder paint does not necessarily have sufficient physical properties such as solvent resistance, adhesion and the like.
In a technique like this by using no curing catalyst, crosslinking formation in forming a film is tried to be improved to attain low temperature melting/low temperature curing by increasing the amounts of functional groups (glycidyl groups in acrylic resin component; carboxyl groups in curing agent component) of a powder paint composition, however, the same problems as described above and the other problems are caused in this case.
Further, Japanese Patent Application Laid-Open (JP-A) No. Hei 5-112743(1993) discloses a powder paint comprising a resin component containing an acrylic resin synthesized from a system containing at least 20% by weight of a monomer containing a glycidyl group and 30 to 50% by weight of a styrene monomer. However, when the amount of styrene is over 30% by weight, the weatherability of a film deteriorates.
Further, Japanese Patent Application Laid-Open (JP-A) No. Hei 5-132634(1993) discloses a technique to form a film according to the same curing method as the prior art from a copolymer as an acrylic resin component synthesized from a system comprising a monomer containing a glycidyl group and tert-butyl acrylate or tert-butyl methacrylate, using as a curing agent component the same compounds as described in the above-described Japanese Patent Application Laid-Open No. Hei 5-112743. However, when tert-butyl acrylate or tert-butyl methacrylate is used as a monomer in an acrylic resin component, these may cause a side reaction with the other monomer having a glycidyl group and may be thermally decomposed when polymerized or desolvated in recovering, and a gel material which is a by-product of a unpreferable side reaction tends to be formed. When a film is formed from a powder paint composition containing such a gel material, an agglomerate tends to be formed non-uniformly on the surface of the film.
Further, U.S. Pat. Nos. 3,919,346 and 3,919,347 disclose a method to crosslink and cure a copolymer composed of a system containing a monomer having a glycidyl group and a monomer having a hydroxyl group as an acrylic resin component, using as a curing agent component an anhydride of a dicarboxylic acid. However, in this invention, storage stability of a paint is poor since the resin component having a glycidyl group and the curing agent composed of an anhydride of a dicarboxylic acid are used.
[Acrylic Resin-based Powder Paint Containing a Compound Having an Acid Anhydride as Curing Agent]
The technique disclosed in Japanese Patent Application Laid-Open (JP-A) No. 50-51542 is a method in which a copolymer constituted from a system containing 5 to 20% by weight of a monomer having a glycidyl group as an acrylic resin component is crosslinked and cured using a curing agent composed of a dicarboxylic acid or a linear acid anhydride. However, when a copolymer containing not more than 20% by weight of a monomer having a glycidyl group is used, the resulting film has insufficient crosslinking density and poor solvent resistance and weatherability.
To solve the problems of the above-described known techniques, a lot of investigations and developments have been carried out. U.S. Pat. No. 4,091,048 and Japanese Patent Application Publication (JP-B) No.58-2983(1983) disclose a paint obtained by crosslinking and curing 5 to 20% by weight of a copolymer containing a monomer having a glycidyl group and an acid anhydride. However, because the copolymer contains not more than 20% by weight of a monomer having a glycidyl group is used, the resulting film has insufficient crosslinking density, and the solvent resistance and weatherability are not necessarily sufficient all the same.
Especially, Japanese Patent Application Publication No. 58-2983 discloses a technique concerning a powder paint composition using a cyclic acid anhydride (a compound having a cyclic acid anhydride group) as a curing agent. Herein, the cyclic acid anhydride may be an aromatic compound or alicyclic compound. However, when the cyclic acid anhydride is used as a curing agent component, there are problems concerning low compatibility between the resin component and the curing agent component and low crosslinking efficiency between the resin component and the curing agent component.
EP 299,420 discloses a powder coating composition containing a modified polyacid anhydride curing agent, which is obtained by using an acrylic copolymer containing a glycidyl group, and a polyol modified polymeric polyacid anhydride wherein polymer chain has an acid anhydride bond and a plurality of acid functional groups and essentially no halogen atom, as a curing agent. However, when the polyol modified polymeric polyacid anhydride is used as a curing agent, hardness in baking is not sufficient and physical properties of the film such as solvent resistance and mar resistance are insufficient, and further, storage stability and the like of a paint are also insufficient.
EP 695,771 discloses a technique using a dicarboxylic acid (poly)anhydride modified with a polyisocyanate as a curing agent for an epoxy functional paint resin. However, when the curing agent is used, resistances such as weatherability and the like of the film are insufficient.
[Technical Background of Acrylic Resin-based Powder Paint Containing Curing Catalyst]
Japanese Patent Application Laid-Open (JP-A) No. 63-165463(1988) discloses a thermosetting acrylic resin powder paint composition mainly composed of a specific glycidyl group functional acrylic resin, aliphatic dibasic acid (anhydride) and alkyl titanate compound, which is melted and cured at low temperature and provides a film having excellent hardness, impact resistance, flex resistance and the like. Namely, there is disclosed a thermosetting acrylic resin powder paint mainly comprising (A) a glycidyl group-functional acrylic resin obtained by copolymerization of an alkyl ester of (meth)acrylic acid having 1 to 14 carbon atoms in the alkyl moiety with a glycidyl ester of (meth)acrylic acid as main components, (B) an aliphatic dibasic acid (preferably, adipic acid, sebacic acid, decanedioic acid, muconic acid and the like) or a linear acid anhydride thereof and (C) an alkyl titanate compound represented by the formula Ti(OR).sub.4 (wherein, R is an alkyl group having 15 to 20 carbon atoms) (for example, tetrapentadecyl titanate and the like).
Herein, the alkyl titanate compound is considered to have a function to catalyze crosslinking formation reaction between a resin component and a curing agent component at low temperature and has an effect to significantly manifest low temperature melting property/low temperature curing property when a film is formed from a powder paint composition. However, in the present invention, low temperature melting property/low temperature curing property, storage stability and the like are not necessarily sufficient.
Japanese Patent Application Laid-Open (JP-A) No. Hei 8-231893 discloses a thermosetting powder paint composition comprising (A) a glycidyl group-containing acrylic copolymer, (B) an aliphatic polyvalent carboxylic acid, and (C) a linear acid anhydride of an aliphatic polyvalent carboxylic acid, or further (D) a salt of a tert-amine compound with an organic acid and/or (E) a tert-amine compound having a melting point from 20 to 150.degree. C.
However, also in this invention, storage stability and low temperature curing ability can not be admitted as sufficient. And when a salt of a tert-amine compound with an organic acid and/or a tert-amine compound having a melting point from 20 to 150.degree. C. as a curing agent is used in a powder paint composition in which an aliphatic polyvalent carboxylic acid coexists as disclosed in Japanese Patent Application Laid-Open No. Hei 8-231893, under storage conditions of relatively high temperature and high humidity including a temperature of about 40.degree. C. and a relative humidity of about 90%, there occurs problems such as gelling and poor appearance of a film due to high reactivity of a glycidyl group with a carboxyl group in storage. Further, there are problems such as remarkable yellowing of a film when baking is conducted at a more strict temperature than usual baking condition.